The celebrated Brazilian architect whose flowing designs infused Modernism with a new sensuality and captured the imaginations of generations of architects around the world, died on Wednesday in Rio de Janeiro. He was 104.
Mr. Niemeyer was among the last of a long line of Modernist true believers who stretch from Le Corbusier and Mies van der Rohe to the architects who defined the postwar architecture of the late 1940s, ’50s and ’60s. He is best known for designing the government buildings of Brasília, a sprawling new capital carved out of the Brazilian savanna that became an emblem both of Latin America’s leap into modernity and, later, of the limits of Modernism’s utopian aspirations. Shielded from the sun behind rows of elegant baffles, the building had a clean, stripped-down style that made it a sparkling example of classical Modernism while heralding Brazil’s emergence as a vibrant center of experimentation.

Mr. Niemeyer’s name soon became synonymous with the new Brazilian architecture. In 1939, he collaborated with Costa on the Brazilian Pavilion for the New York World’s Fair. Three years later, he completed his first house, a simple modern box resting on slender columns on a mountainside overlooking the magnificent Rodrigo de Freitas lagoon. In these and other early projects, Mr. Niemeyer was beginning to develop a distinctive architecture of flowing lines, structural lightness and an open relationship to natural surroundings.

Mr. Niemeyer’s international status was confirmed by the Brazil Builds exhibition at the Museum of Modern Art in New York in 1943, a show that also introduced his work to an American audience. Four years later, he joined Le Corbusier again, this time as an equal, when the two were selected to take part in designing the United Nations complex in Manhattan.
Supervised by Wallace K. Harrison, the United Nations design was a collaboration that also included international luminaries like the Soviet architect Nikolai D. Bassov and Max Abramovitz of New York. The final design was a compromise of sorts between Mr. Niemeyer’s concepts and those of his aging idol Le Corbusier.
Set amid gardens and plazas, the slim, glass-clad Secretariat tower and the sculptural concrete General Assembly building remain testaments to the belief in rationalism as a means to resolve international disputes and disparities.

In celebrating both the formal elements and social aims of architecture, his work became a symbolic reminder that the body and the mind, the sensual and the rational, are not necessarily in opposition. Yet he also saw sensuality and the brightness of dreams against a darker backdrop. “Humanity needs dreams to be able to survive the miseries of daily existence,” he once said, “even if only for an instant.”